Fine structure of the chorion and site of sperm entry in the egg of <i>Brachydanio</i>

Abstract

Abstract The fine structure of the chorion and the region of the unfertilized egg immediately beneath the micropylar apparatus of the zebra danio, Brachydanio rerio , were studied using Nomarski differential interference optics, and scanning and transmission electron microscopy. The chorion consisted of three distinct zones: an outer, electron‐dense zone containing pore canal plugs (zona radiata externa), a middle fibrillar zone (superficial zona radiata interna), and an inner zone of 16 horizontal electrondense lamellae alternating with 15 interlamellae of lower electron density (deep zona radiata interna). The zona radiata interna was pierced by open pore canals. The single micropylar apparatus was regionalized into a cone‐shaped vestibule and a tapered micropylar canal traversing the entire chorion. The outer diameter of the micropylar canal was 7.5–8.5 μn and the inner diameter about 2.3 μn. Since the diameter of the inner micropylar aperture was slightly larger than the size of the sperm head, the block to polyspermy in eggs of the zebra danio appears to be mechanical and guaranteed by the morphological design of the micropyle. The egg plasmalemma beneath the inner micropylar aperture was differentiated as a circular cluster of 15–20 microvilli‐like projections. The cluster of surface projections, approximately 2.1–2.5 μn in diameter under scanning, electron microscopy, was distinguishable from the microplicae covering the rest of the egg surface and identified as the sperm entry site. The cortical cytoplasm subjacent to the sperm entry site was organized as a compact, electron‐dense, and homogeneous band. The sperm entry site itself was circumscribed by an area of cytoplasm (approximately 100 μn in diameter) in which cortical granules were typically arranged as a single layer immediately beneath the plasmalemma. However, there was a complete absence of cortical granules in the cytoplasm directly below the sperm entry site. The single row arrangement contrasted with the multilayered rows of cortical granules found throughout the remainder of the egg cytoplasm. Based upon Nomarski and ultrastructural analyses, there was a significant polarity in the cortex created by the size distribution and volume density of the cortical granules layered just beneath the plasmalemma. The cortical granules in the vicinity of the sperm entry site were 2.7 to 2.8 μm in diameter and densely packed. From this region to the vegetal pole, the cortical granules appeared to progressively increase in size and become less densely packed. The polarity in granule distribution established a distinct gradient in the structural organization of the egg cortex from the site of sperm entry to the vegetal pole.